1. Field of the Invention
The field of this invention relates to control systems for robotic manipulators and more particularly relates to adaptive force and position controllers combined in a hydrid control system.
2. Background of the Invention
Hybrid control systems, i.e., those systems which together supply signals to control the position of, and force exerted by, a robotic manipulator simultaneously are known. Such systems are required because most manipulators are operated in the presence of environmental constraints, and certain degrees-of-freedom are lost for motion due to the environmental constraints. For example, when the manipulator makes contact with the environment, the contact forces must be controlled in the constraint directions, while the positions are controlled simultaneously in the free directions.
The problem of manipulator control in a constrained environment has been investigated by several researchers. See, for example D. E. Whitney, "Historical Perspective and State of the Art in Robot Force Control," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 262-268, St. Louis, MO, 1985. At present, three major conceptual approaches exist for simultaneous position and force control. Paul and Shimano (R. P. Paul and B. Shimano, "Compliance and Control," Proc. Joint Aut. Control Conf., pp. 694-699, San Francisco, CA, 1976) suggest a method which uses certain joints for position control while the remaining joints are used for force control. Salisbury (J. K. Salisbury, "Active Stiffness Control of a Manipulator in Cartesian Coordinates," Proc. IEEE Conf. on Decision and Control, pp. 95-100, Albuquerque, NM, 1980) puts forward a technique for controlling the end-effector stiffness characteristics in the Cartesian space. Raibert and Craig (M. H. Raibert and J. J. Craig, "Hybrid Position/Force Control of Manipulators," ASME J. Dyn Systems, Measurement and Control, Vol. 102, pp. 126- 133, 1981) proposed a conceptual architecture, based on the analysis of Mason for "hydrid control" (M. T. Mason, "Compliance and Force Control for Computer Controlled Manipulators," IEEE Trans. Systems, Man and Cybernetics, SMC11(6), pp. 418-432, 1981). Hybrid control allows forces to be controlled in the constraint directions by a force controller, while simultaneously controlling positions in the free direction by a position controller. Raibert and Craig, however, do not prescribe a general and systematic method for the design of position and force controllers. Nevertheless, hybrid control has gained considerable popularity over the other two alternatives for simultaneous position and force control. Hybrid control has been used extensively in the literature. See, for example, H. Zhang and R. P. Paul, "Hybrid Control of Robot Manipulators," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 602-607, St. Louis, MO, 1985; H. West and H. Asada, "A Method for the Design of Hybrid Position/Force Controllers for Manipulators Constrained by Contact with the Environment," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 251-259, St. Louis, MO, 1985; P. G. Backes, G. G. Leininger and C. H. Chung, "Real-Time Cartesian Coordinate Hybrid Control of a PUMA 560 Manipulator," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 608-613, St. Louis, MO, 1985; P. G. Backes, G. G. Leininger and C. H. Chung, "Joint Self-Tuning with Cartesian Setpoints," ASME J. Dyn. Systems, Measurement and Control, Vol 108, pp. 146-150, 1986; O. Khatib and J. Burdick, "Motion and Force Control of Robot Manipulators," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 1381-1386, San Francisco, CA, 1986; T. Yoshikawa, "Dynamic Hybrid Position/Force Control of Robot Manipulators," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 1393-1398, San Francisco, CA, 1986; A. J. Koivo, "Force-Position-Velocity Control with Self-Tuning for Robotic Manipulators," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 1563-1568, San Francisco, CA, 1986; C. C. Nguyen, F. J. Pooran and T. Premack, "Control of Robot Manipulator Compliance," Proc. Intern. Symp. on Robot Manipulators, Albuquerque, NM, 1986; O. Khatib, "Dynamic Control of Manipulators in Operational Space," Proc. 6th IFToMM Congress on Theory of Machines and Mechanisms, pp. 1128-1131, New Delhi, India, 1983 (Whiley, New Delhi); S. D. Eppinger and W. P. Seering, "On Dynamic Models of Robot Force Control," Proc. IEEE Intern. Conf. on Robotics and Automation, pp. 29-34, San Francisco, CA 1986; and J. W. Gilbart, R. V. Monopoli and C. F. Price, "Improved Convergence and Increased Flexibility in the Design of Model Reference Adaptive Control Systems," Proc. IEEE Conf. on Decision and Control, pp. IV 3.1-3.10, Austin, TX, 1970.
Position sensors and force sensors are well known and actually measure the position and force at a controlled end effector. The end effector behavior, whether position alone, or position and force, results from voltages applied to motors that run the joints in a robotic arm. In Raibert and Craig, the hybrid control system requires coordinate transformations of the measurements to develop a feedback term, which through very extensive inverse Jacobian computations, controls the motors in the joints of the robotic arm by independent servo control loops. Such an approach requires knowledge of those motors parameters and other robotic arm parameter values, as well. Furthermore, independent servo control loops, as suggested by Raibert and Craig, do not accommodate cross coupling of position and force.
This invention, in distinction from Raibert and Craig, is a truly adaptive controller for force and position. Since the adaptive controller senses force and position in Cartesian space and formulates driving voltages which achieve the Cartesian force and position, advance knowledge of the robotic arm, joint motor parameter and environment values is not necessary. A highly stable and adaptive real-time controller is achieved by this invention.